Control system for sulfur process

ABSTRACT

The flow rate of the oxygen-containing gas to a process for the oxidation of the hydrogen sulfide in a sour gas to sulfur is regulated responsive to the lower of the sour gas pressure signal and a signal representing the value of the sour gas flow rate as modified by the ratio of hydrogen sulfide to sulfur dioxide in the gaseous reaction effluent, to maintain the desired ratio of hydrogen sulfide to oxygen fed to the process. The sour gas flow rate can be regulated responsive to the higher of the sour gas pressure signal and the oxygen-containing gas flow rate signal.

This is a division of our copending application Ser. No. 526,939, filedon Nov. 25, 1974, now U.S. Pat. No. 3,985,864.

This invention relates to the production of sulfur from hydrogensulfide. In a specific aspect the invention relates to a control systemfor a process for oxidizing hydrogen sulfide to sulfur.

In the stoichiometric control of the mixing of a sour gas and anoxygen-containing gas in a process for the oxidation of the hydrogensulfide content of the sour gas to elemental sulfur, we have five basicobjectives. The first objective is to maintain the quantity of oxygenbelow that stoichiometrically required for the oxidation of the hydrogensulfide in order to prevent the formation of sulfates in the processeffluent. The production of sulfates in the gaseous effluent can causeoperating problems in downstream processing equipment. The secondobjective is to maintain the oxygen quantity as close as possible to thestoichiometrical requirement in order to promote the highest possibleefficiency in oxidizing the sour gas and to reduce the sulfur content ofthe gaseous effluent from the process. The third objective is tomaintain stable control of the process while achieving the first twoobjectives even though the sour gas flow rate may vary. The fourthobjective is to maintain stable control of the process while achievingthe first two objectives even though the hydrogen sulfide content of thesour gas may vary. The fifth objective is to effect stable control ofthe process while achieving the first four objectives even though thereis a time delay between the ocurrence of a variation in one or both ofthe process feed streams and the occurrence of the measurement of theeffect of that variation on the gaseous effluent from the process. Otherobjects, aspects and advantages of the invention will be apparent from astudy of the specification, the drawing and the appended claims to theinvention.

In the drawing, the single FIGURE is a schematic representation of asystem for producing sulfur from hydrogen sulfide in accordance with oneembodiment of the present invention.

Sour gas, comprising a significant concentration of hydrogen sulfide, ispassed through conduit 11 to the fuel inlet of boiler 12. A suitableoxygen-containing gas, such as air, is passed through conduit 13 to theair inlet of boiler 12. Water is introduced into an enclosed chamber 14in boiler 12 by way of conduit 15. The water in chamber 14 is inindirect heat exchange relationship with the hot combustion gasesresulting from the reaction in the boiler 12 of the oxygen and thehydrogen sulfide being fed to the boiler by way of conduits 13 and 11,respectively. Thus, the water in chamber 14 is converted into steam,which is withdrawn by way of conduit 16. The formation of the steam inchamber 14 results in the cooling of the hot combustion gases withinboiler 12. The combustion products from boiler 13 normally comprisesulfur dioxide, unreacted hydrogen sulfide, water vapor, carbon dioxide,nitrogen, and a small amount of sulfur. These combustion products arewithdrawn from the boiler 12 and passed via conduit 17 into phaseseparator 18 for the separation of the liquid sulfur therefrom. Theseparated liquid sulfur is passed via conduit 19 into sulfur storage 20,while the gaseous combustion products are passed via conduit 21 intoreactor 22. The sulfur dioxide is reacted with the hydrogen sulfide inreactor 22 to form sulfur. Reactor 22 can contain any suitable catalystknown in the art, for example, bauxite. The hot reaction effluent fromreactor 22 is passed through conduit 23, cooler 24 and conduit 25 intophase separator 26. Liquid sulfur is withdrawn from phase separator 26and passed via conduit 27 into sulfur storage 20. The remaining gaseouseffluent is withdrawn from separator 26 and passed via conduit 28 intocoalescer 29. Liquid sulfur is withdrawn from the bottom portion ofcoalescer 29 and passed via conduit 31 into storage 20, while thestripped gas is withdrawn from coalescer 29 via conduit 32.

The pressure of the air in conduit 13 is measured by a pressure sensor37 and a signal representative of the thus measured pressure is appliedto the measurement input of pressure controller 38. A signal 39,representing the desired air pressure in conduit 13, is applied to thesetpoint input of controller 38. The output of controller 38 isresponsive to the difference between the measurement signal and thesetpoint signal and is applied to the control input of motor valve 40,operatively connected in conduit 13 upstream of sensor 37.

The pressure of the sour gas in conduit 11 is measured by sensor 41 anda signal representative of the thus measured pressure is applied to themeasurement input of pressure controller 42. Controller 42 compares thepressure measurement signal with a setpoint signal 43, representing thedesired sour gas pressure, and produces an output signal 44 responsiveto the difference between the measured pressure and the desiredpressure. Signal 44 is applied to one input of high select relay 45.

A signal 46 representative of the square of the flow rate of air throughconduit 13 is produced by an orifice meter 47, located in conduit 13downstream of valve 40, and passed to square root extractor 48. Theoutput signal 49 from square root extractor 48 is proportional to theflow rate of air in conduit 13, and is applied to the second input ofhigh select relay 45. Relay 45 compares the two input signals 44 and 49applied thereto and passes the higher of the two as output signal 51,which is then applied to the setpoint input of flow controller 52. Asignal 53 representative of the square of the flow rate of sour gasthrough conduit 11 is produced by an orifice meter 54 and passed tosquare root extractor 55. The output signal 56 from square rootextractor 55 is proportional to the flow rate of sour gas in conduit 11,and is applied to the measurement input of flow controller 52.Controller 52 compares the measurement input signal 56 and the setpointsignal 51 and produces an output signal 57 responsive to the differencebetween the measurement input signal 56 and the setpoint signal 51. Theoutput signal 57 is applied to the control input of motor valve 58 whichis operatively connected in conduit 11 downstream of pressure sensor 41.

An analyzer ratio computer 61 is operatively connected to conduit 32 toobtain a series of samples of the gases flowing through conduit 32,analyze each sample for the concentration therein of hydrogen sulfideand of sulfur dioxide, and produce an output signal 62 representative ofthe ratio of the concentration of hydrogen sulfide to the concentrationof sulfur dioxide in the current sample. Ratio signal 62, which is inelectrical form, is applied to current-to-pressure transducer 63 toproduce a pneumatic signal 64 representative of the current ratio. Thesignal 64 is applied to lag device 65 to smooth out the abrupt changesin the ratio signal 64. The resulting lagged signal 66 is applied to oneinput of high select relay 67. A manually regulatable air pressuresignal 68, provided by pressure regulator 69 and air supply 71, isapplied to the second input of high select relay 67. Relay 67 comparessignals 66 and 68 and passes the higher thereof as input signal 72 tothe A input of analog unit 73. Signal 56, representative of the flowrate of sour gas in conduit 11, is applied to the B input of analog unit73. A manually regulatable pressure signal 74, provided by air supply 75and pressure regulator 76, is applied to the C input of analog unit 73.The relationship of output signal 77 of analog unit 73 to the inputsignals to analog unit 73 is defined by:

    P = B + K (A-C)

wherein

P is the output signal 77,

A is the input signal 72 which is applied to the A input port,

B is the input signal 56 which is applied to the B input port,

C is the input signal 74 which is applied to the C input port, and

K is 100/G, where G is the gain of the analog unit 73.

Once the gain of unit 73 is selected, K becomes a constant. The outputsignal 77 is taken from the P port of analog unit 73 and applied to oneinput of low select relay 78. The signal 44, responsive to thedifference between the measured pressure in conduit 11 and the desiredsour gas pressure, is applied to the second input of relay 78. Relay 78compares signals 44 and 77 and passes the lower thereof as the setpointsignal 79 for flow controller 81. The air flow signal 49 is applied tothe measurement input of flow controller 81. The output signal 82 fromflow controller 81, which is responsive to the difference betweensignals 49 and 79, is applied to the control input of motor valve 83which is operatively connected in conduit 13.

The sour gas flow rate orifice meter 54 and the air flow rate orificemeter 47 can be sized in such a relationship that the pneumatic signal56 representing the normal flow rate of hydrogen sulfide through conduit11 (based on the normal, or average, concentration of hydrogen sulfidein the sour gas) equals the pneumatic signal 49 for the correspondingflow rate of oxygen through conduit 13 (based on the normal, or average,concentration of oxygen in the oxygen-containing gas passing throughconduit 13) necessary to convert the desired portion, generallyone-third, of the hydrogen sulfide in the sour gas stream to sulfurdioxide. The bias on pressure controller 42 can also be adjusted so thatwhen the measured pressure equals the desired pressure represented bysetpoint signal 43 the pneumatic signal 44 equals the equilibrium valuesof pneumatic signals 56 and 49. In other words, at equilibriumconditions, signal 72 equals signal 74, signal 77 equals signal 56, andsignal 44 equals signal 49 and 77.

Assume that the control system is in equilibrium and then the pressureof the sour gas in conduit 11 increases because of some variable in thesource of the sour gas. The increased sour gas pressure will be measuredby pressure sensor 41, causing the pneumatic signal 44 to increase overits equilibrium value. At this point in time, signal 44 will be greaterthan signals 49 and 77, causing low select relay 78 to pass signal 77 toair flow controller 81, and high select relay 45 to pass signal 44 tosour gas flow controller 52. The increase in the setpoint signal 51 toflow controller 52 causes valve 58 to open further, thereby increasingthe flow rate of the sour gas. The increased sour gas flow rate isdetected by orifice flow meter 54, resulting in an increase in the valueof signal 56 being applied to the B input of analog unit 73. Even thoughthe value of signal 77 is increased as a result of the initial increasein sour gas flow rate signal 56, the value of signal 77 will still belower than the value of signal 44, so that low select relay 78 willcontinue to pass the increased signal 77 to the setpoint of the air flowcontroller 81, thereby increasing the air flow rate. The increase in airflow rate is sensed by flow rate orifice meter 46, and the value ofsignal 49 to high select relay 45 is increased accordingly. In themeantime, the sour gas pressure will decrease as a result of theincreased flow rate of sour gas, resulting in a corresponding decreasein sour gas pressure signal 44. This continues until the air flow valuesignal 49 slightly exceeds sour gas pressure signal 44, thereby causinghigh select relay 45 to pass the air flow rate signal 49 to the input ofsour gas flow controller 52, increasing the flow of sour gas and thevalue of signal 56 to the point where signal 77 slightly exceeds signal44, thereby causing low select relay 78 to pass the lower signal 44 toair flow controller 81 to reduce the air flow rate and the correspondingair flow rate signal 49. Thus the system reaches equilibrium at the new,higher values of flow rates of air and sour gas, with signals 44, 77 and49 again being equal.

If the sour gas pressure decreases because of some variable in thesource of sour gas, the opposite actions occur. The sour gas pressuresignal 44 will be passed by low select relay 78 to air flow controller81 to reduce the air flow rate. The decreasing value of the air flowrate signal 49 is passed by high select relay 45 to the sour gas flowcontroller 52 to decrease the sour gas flow rate accordingly. Thedecreasing value of the sour gas flow rate signal 56 causes acorresponding decrease in the signal 77. Meanwhile the reduction in sourgas flow rate causes the sour gas pressure to increase. This continuesuntil the system again reaches equilibrium. Thus, the desired ratio ofhydrogen sulfide to oxygen is maintained despite changes in sour gaspressure.

If any variation occurs in the hydrogen sulfide concentration in thesour gas stream or in the oxygen concentration in the air stream orboth, the resulting change in the ratio of hydrogen sulfide to sulfurdioxide in effluent gas stream 32 is detected by analyzer ratio computer61 and signal 66 is varied accordingly. Signal 68 is employed as a lowerlimit for the control range of the measured ratio so that high selectrelay 67 passes the lagged ratio measurement signal 66 so long as it isabove the lower limit of the control range. If the lagged ratiomeasurement signal 66 equals the signal 74, which represents the desiredratio, the output signal 77 is equal to the sour gas flow rate signal56. If the lagged ratio measurement signal 66 is greater than signal 74,the output signal 77 of the analog unit 73 will be greater than the sourgas flow rate signal 56, and will also be greater than the sour gaspressure signal 44 if the system is otherwise at equilibrium. Thus, lowselect relay 78 will pass the sour gas pressure signal 44 to air flowcontroller 81 to control the air flow rate responsive to the sour gaspressure. However, if the lagged signal 66 representing the measurementof the ratio of hydrogen sulfide to sulfur dioxide becomes less than thevalue of standard signal 74, the output signal 77 of analog unit 73 willbecome less than the value of the sour gas flow rate signal 56, and alsoless than the value of the sour gas pressure signal 44 if the system isotherwise at equilibrium. Low select relay 78 will pass the lower analogunit output signal 77 to air flow controller 81 to reduce the air flowrate until the lagged measured ratio of hydrogen sulfide to sulfurdioxide again equals the desired ratio represented by standard signal74.

Any suitable components known in the art can be employed to constructthe control system of this invention, including electrical components,pneumatic components, mechanical components, and combinations thereof,with either analog or digital output signals. In one embodimentutilizing pneumatic components, other than the electronic analyzer ratiocomputer, the following components were employed:

pressure controller 42: Foxboro 5422TS58P4 recorder controller

square root extractors 48 and 55: Foxboro 557 square root extractors

flow controllers 52 and 81: Foxboro 5422PS58P4 recorder controllers

relays 45 and 67: Foxboro B-114-YL relays

relay 78: Foxboro B-114-BZ relay

analog unit 73: Foxboro 56-1

lag 65: Foxboro adjustable restrictor lag -- B102RP

analyzer ratio computer 61: Applied Automation, Inc., Model 102 Analyzer

transducer 63: Fisher 1/P transducer 546

valves 83 and 58: Fisher control valve (air to open)

The valve 40 was not employed as the air source was a turbine-drivencompressor with the turbine speed being regulated responsive to thecompressed air pressure to maintain a constant discharge pressure. Thevalue of 2.3 was selected for the desired ratio of hydrogen sulfide tosulfur dioxide in conduit 32. This value of 2.3 was represented by apnuematic pressure of 12.3 psig. Regulator 69 was adjusted to provide apnuematic signal 68 representative of a minimum ratio of 1.8. The gain Gof analog unit 73 was set at 135. The integration time in lag 65 was setat 6 minutes.

In one embodiment of the invention, it was desirable to feed the sourgas through conduit 11 at the maximum rate available. The output signal51 was isolated from flow controller 52 by means of an auto-manualselector switch, and the setpoint of flow controller 52 was manuallybiased to maintain valve 58 at the fully opened condition, therebyeffectively negating the function of high select relay 45. The sameeffect could have been achieved by simply omitting high select relay 45,flow controller 52 and value 58. The sour gas was passed through conduit11 as available, while flow controller 81 regulated the air flow rateresponsive to the lower of the sour gas pressure signal 44 and theanalog unit output signal 77, thereby maintaining the desired ratio ofhydrogen sulfide to oxygen entering boiler 12.

Reasonable variations and modifications are possible within the scope ofthe foregoing disclosure, the drawing and the appended claims to theinvention.

That which is claimed is:
 1. Apparatus for the production of sulfurcomprising means defining a combustion chamber, first conduit means forpassing a first stream containing hydrogen sulfide into said combustionchamber, second conduit means for passing a second stream containingoxygen into said combustion chamber to therein convert a portion of thehydrogen sulfide of said first stream to sulfur dioxide, a reactor,third conduit means for withdrawing the resulting combustion productsfrom said combustion chamber and passing the withdrawn combustionproducts to said reactor, fourth conduit means for withdrawing thereaction effluent from said reactor, means for separating the thuswithdrawn reaction effluent into a liquid sulfur stream and a strippedgas stream, means for measuring the pressure of said first stream andestablishing a first signal representative thereof, means for measuringthe flow rate of said first stream and establishing a second signalrepresentative thereof, means for determining the ratio of theconcentration of hydrogen sulfide in said stripped gas stream to theconcentration of sulfur dioxide in said stripped gas stream andestablishing a third signal representative thereof, means forestablishing a fourth signal responsive to said second and thirdsignals, and means for controlling the rate of flow of said secondstream responsive to the lower of said first and fourth signals tomaintain the desired ratio of hydrogen sulfide to oxygen fed to saidcombustion chamber despite any changes in the pressure of said firststream and any variation in the concentration of hydrogen sulfide insaid first stream and in the concentration of oxygen in said secondstream.
 2. Apparatus in accordance with claim 1 further comprising meansfor measuring the flow rate of said second stream and establishing afifth signal representative thereof, and means for controlling the flowrate of said first stream responsive to the higher of said first andfifth signals.
 3. Apparatus in accordance with claim 2 wherein saidmeans for establishing a fourth signal comprises means for establishinga sixth signal representative of the desired ratio of the concentrationof hydrogen sulfide in said stripped gas stream to the concentration ofsulfur dioxide in said stripped gas stream, means for establishing aseventh signal responsive to the difference between said third signaland said sixth signal, and means for producing said fourth signalresponsive to the algebraic sum of said second signal and said seventhsignal.
 4. Apparatus in accordance with claim 3 wherein said means fordetermining the ratio and establishing a third signal comprises meansfor establishing an eighth signal proportional to the determined ratioof the concentration of hydrogen sulfide in said stripped gas stream tothe concentration of sulfur dioxide in said stripped gas stream, and lagmeans for producing said third signal as a lagged function of saideighth signal.
 5. Apparatus in accordance with claim 4 furthercomprising means for maintaining the pressure of said second streamsubstantially constant.
 6. Apparatus in accordance with claim 2 whereinsaid means for establishing a fourth signal comprises means forestablishing a sixth signal representative of the desired ratio ofconcentration of the hydrogen sulfide in said stripped gas stream to theconcentration of sulfur dioxide in said stripped gas stream, an analogdevice having the function P = B + K (A--C) wherein P is said fourthsignal, B is said second signal, A is said third signal, C is said sixthsignal and K is a constant, and means for applying said second, thirdand sixth signals to said analog device.
 7. Apparatus in accordance withclaim 6 wherein said means for establishing a third signal comprises ananalyzer-computing means for analyzing a sample of said stripped gasstream to determine the concentration of hydrogen sulfide and of sulfurdioxide therein and to produce a seventh signal proportional to theratio of the determined concentration of hydrogen sulfide in saidstripped gas stream to the determined concentration of sulfur dioxide insaid stripped gas stream, and lag means for producing said third signalas a lagged function of said seventh signal.
 8. Apparatus in accordancewith claim 1 wherein said means for establishing a fourth signalcomprises means for establishing a fifth signal representative of thedesired ratio of the concentration of the hydrogen sulfide in saidstripped gas stream to the concentration of sulfur dioxide in saidstripped gas stream, an analog device having the function P = B + K(A--C) wherein P is said fourth signal, B is said second signal, A issaid third signal, C is said fifth signal and K is a constant, and meansfor applying said second, third and fifth signals to said analog device.9. Apparatus in accordance with claim 1 wherein said means forestablishing a third signal comprises an analyzer-computing means foranalyzing a sample of said stripped gas stream to determine theconcentration of hydrogen sulfide and of sulfur dioxide therein and toproduce a fifth signal proportional to the ratio of the determinedconcentration of hydrogen sulfide in said stripped gas stream to thedetermined concentration of sulfur dioxide in said stripped gas stream,and lag means for producing said third signal as a lagged function ofsaid fifth signal.
 10. Apparatus in accordance with claim 1 wherein saidmeans for establishing a fourth signal comprises means for establishinga fifth signal representative of the desired ratio of the concentrationof hydrogen sulfide in said stripped gas stream to the concentration ofsulfur dioxide in said stripped gas stream, means for establishing asixth signal responsive to the difference between said third signal andsaid fifth signal, and means for producing said fourth signal responsiveto the algebraic sum of said second signal and said sixth signal.